3.10 The untrue fungi

In our discussion so far, we have sometimes used the
description ‘true fungi’ to refer to those organisms that belong to Kingdom
Fungi. This implies that there are some organisms that might be called
‘untrue fungi’ because they look like fungi but are not fungi. And, well,
yes this is the case (Rossman & Palm, 2006).

Water moulds. This is an informal grouping that
includes the most ancient fungi and fungus-like organisms:

The Chytridiomycota, water
moulds that are the ancestral group of the true fungi (Kingdom Fungi)
and which we have already discussed [CLICK
HERE if you need a reminder].

Oomycota & Hyphochytriomycota - water
moulds that are not true fungi, but have affinities to the algae. They are now
placed in Kingdom Chromista (as subphylum
Pseudofungi; Cavalier-Smith, 2018), or (our preference)
Kingdom Straminipila (Beakes et al., 2014).

Phylum Oomycota(Beakes
et al., 2014)consists of about 600 species in 90 genera, placed in the
following Orders:

Base pair at the very base of helix 47 in the VA regions
of the 18S rRNA sequences

AU

UA

Although these are primitive organisms, at least in terms of their
evolutionary position with respect to Fungi, they are highly adapted to their
life style. This is illustrated by the behaviour of zoospores
of Oomycota.

Temperature influences the fate of the zoosporangium in Phytophthora
infestans (cause of Potato Blight): below 15°C the zoosporangium forms
zoospores, but above 20°C it forms a germ tube (so in the cold water of the
soil the zoospores will swim to find new hosts; in the warm sunshine the
sporangium will infect the plant.

After release, zoospores typically swim for many hours; zoospores of
Phytophthora megasperma swim at 88 µm sec-1 at 15°C; so it
only takes them 11 seconds to cover a distance of 1 mm.

Zoospores may show amoeboid movement when in contact with a solid
substratum; allowing slow-speed targeting on the host.

Zoospores show tactic movements. A ‘taxis’ is a movement towards or
away from a stimulus (tropism is growth towards or away from a stimulus).

Zoospores of Phytophthora palmivora are negatively geotactic
(they swim upwards; which is where the nice newly-formed host leaves and
buds will be found).

Zoospores of Pythium aphanidermatum have a positive chemotaxis
to roots.

Hyphae of Oomycota are also positively chemotropic (hyphae
of true fungi are not chemotropic, although they do exhibit other tropisms,
particularly autotropism).

Saprolegnia is another important genus. Saprolegnia species are
parasites of freshwater fish and fish eggs, and can cause economic damage on
fish farms. Reproduction is mainly asexual, but the life cycle (Fig. 5) includes
a sexual phase.

Fig. 5. Diagrammatic life cycle of Saprolegnia

Hyphal branches become modified into long zoosporangia
separated from the hypha by septa. Biflagellate zoospores released from a
zoosporangium swim for a while and then encyst. Each eventually gives rise to a
secondary zoospore, which also encysts and then germinates to produce a new
mycelium.

For sexual reproduction, compatible oogonia and
antheridia develop on the same diploid mycelium. Meiosis occurs within
these gametangia. In mating, antheridia grow toward the oogonia and develop
tubular processes called ‘fertilisation tubes’, which penetrate the oogonia.
Male nuclei travel through the fertilisation tubes to fuse with the female
nuclei within (karyogamy). Following karyogamy, a thick-walled zygote, called
the oospore, is produced. The oospore germinates into hyphae, which then produce
a zoosporangium.

Pheromones in Achlya

Achlya bisexualis is heterothallic. Sterols are involved in the
hormonal mechanisms regulating sexual reproduction in Achlya (as they
are in animals).

The female mycelium produces antheridiol, which induces male hyphae to
make antheridial branches.

Antheridial hyphae are attracted by antheridiol (this is a chemotropism)
to the female mycelium.

Antheridia grow around the oogonium, producing fertilisation tubes
(plasmogamy) and enable karyogamy which results in oospore formation. So even
these primitive organisms have a sterol hormone mechanism for cell targetting.

Other fungus-like members of Kingdom Straminipila are:

Phylum Hyphochytriomycota (Beakes et al.,
2014), which are microscopic organisms that form a small thallus, often with
branched rhizoids, occurring as parasites or saprotrophs on algae and fungi
in freshwater and in soil. The whole of the thallus is eventually converted
into a reproductive structure. Only 23 species (in 6 genera) are known and
are placed in Order Hyphochytriales (example genera are: Hyphochytrium,
Rhizidiomyces).

Phylum Labyrinthulomycota (Beakes et al.,
2014), in which the feeding stage comprises an ectoplasmic network and
spindle-shaped or spherical cells that move within the network by gliding
over one another. They occur in both salt- and freshwater in association
with algae and other chromists. There are about 45 species in 10 genera,
placed in the two Orders Labyrinthulales (e.g. Labyrinthula) and
Thraustochytriales (e.g. Thraustochytrium).

Slime moulds. The organisms known as slime moulds are now
all placed in Kingdom Protozoa. They do not form hyphae, and
they generally lack cell walls, being capable of ingesting food particles by
phagocytosis. The slime moulds fail to meet normal definitions of fungi, but
they produce fruiting bodies which have a superficial resemblance to those of
fungi, and this is why they have been called ‘moulds’ and have been studied by
mycologists and included in most textbooks on mycology. They are placed in three
phyla in the
Protozoa:

Order Plasmodiophoromycota (included in the
PhylumPhytomyxea) are obligate intracellular
symbionts or parasites of plant, algal or fungal cells living in freshwater
or soil habitats. They have multinucleate, unwalled plasmodia. There are
about 15 genera with 50 species in the Order Plasmodiophorales; example
genera are
Plasmodiophora, Polymyxa and Spongospora.
Plasmodiophora and Spongospora cause serious plant diseases
(Bulman & Braselton, 2014).

Phylum Myxomycota (in the Supergroup Amoebozoa)
are free-living unicellular or plasmodial amoeboid slime moulds. A total of
900 species assigned to 80 genera, and seven Orders, among which are the
Dictyosteliales (e.g. Dictyostelium), Physarales (e.g. Didymium,
Physarum, Fuligo) and Stemonitales (e.g. Stemonitis)
(Stephenson, 2014).

Phylum Acrasiomycota (in the Supergroup
Excavata) are generally saprotrophic amoeboid slime moulds, found
on a wide range of decaying plant material. A total of 12 species assigned
to 6 genera in the single Order Acrasiales (e.g. Acrasis,
Copromyxa) (Stephenson, 2014).

Finally in the Phylum Choanozoa (Kingdom Protozoa) are
placed two groups of organisms previously misclassified as
trichomycete fungi. They are now placed in
Class Mesomycetozoea making up the Orders Amoebidiales
and Eccrinales. They are intimately associated with arthropods,
insects, millipedes and crustaceans and have a coenocytic thallus attached to
the host by a holdfast.